Coatings are applied to various substrates for a variety of purposes. Although it can be applied to any desired substrates, the present invention and the problems on which it is based are explained in more detail with respect to carbon fiber reinforced plastic components (CRP components) in aerospace. However, the invention is not restricted to the application to carbon fiber reinforced plastic components, but can be applied to many different substrates, in particular fiber reinforced plastics.
Carbon fiber reinforced plastic components are coated for use in an aircraft, for example to provide protection from wear, corrosion protection, lightning protection, corrosion protection, protection from excessive temperature fluctuations or for heat insulating reasons. These coatings may be advantageously applied by thermal spraying. However, other methods of deposition are also conceivable. The invention therefore does not relate exclusively to coating by thermal spraying but can also be applied to other coating methods. It is possible by the method of thermal spraying to apply a wide variety of materials to substrates. For example, most metals or metal alloys allow thermal spraying, as do various ceramics and plastics.
To apply metals or their alloys by thermal spraying, for example a wire or a powder of the metal or of the alloy can be melted in an electric arc (in the case of arc spraying) or an oxygen-fuel flame (in the case of wire or powder flame spraying). Using compressed air or an inert gas, the molten particles are then accelerated at high speed onto the substrate. Another form of thermal spraying includes a plasma as the thermal energy source for (incipient) melting of the spray powder particles (in the case of plasma or high-velocity plasma spraying).
The acceleration of the particles may also take place by burning a combustible gas with oxygen in a confined volume and its expansion through a nozzle, thereby producing a high-velocity stream, into which a powder is introduced by means of a directed gas stream (in the case of high-velocity oxyfuel spraying, HVOF).
The adhesive attachment of the particles is proportional to the energy that these particles possess. The energy may be both in the form of thermal energy, that is to say in the form of heated particles, and in the form of kinetic energy, that is to say in the form of highly accelerated particles. The velocity of the particles may easily reach supersonic speed. In the case of thermal spraying, the particles have both high thermal energy and high kinetic energy. Those particles that are the first to impinge on the surface to be coated form a base for the further layers to be deposited.
One problem encountered in the coating of CRP components, however, is the poor adhesive attachment of the thermally sprayed layers on the surface of the CRP component. In order to compensate for this disadvantage, various methods have been proposed. For example, the surface of the CRP component may be roughened by means of blasting with corundum, in order to achieve better adhesive attachment of the thermally sprayed-on layer. However, this is sometimes accompanied by the fibers being exposed, and this can lead to impairment of the structural-mechanical properties of the component. In addition, if the fibers come into contact with a metallic sprayed layer, galvanic corrosion may occur if an electrolyte is present.
In the case of direct application of a coating by means of thermal spraying or else in the case of application of an adhesion-promoting intermediate layer by means of thermal spraying, the CRP component is also exposed to high thermal loading. This high thermal loading may likewise already lead to impairment of the coated CRP component during its production. Furthermore, when applying a number of layers, the substrate must be cooled between the individual application steps, making the direct coating of CRP components very time-intensive.
The applicant knows of further methods in which, for example, an intermediate layer of resin is applied directly to the CRP component before the final coating is applied to this intermediate layer. Particles intended to serve as an adhesion promoter may also be additionally mixed in with this intermediate layer of resin. Once the resin has cured, part of the intermediate layer is removed again, in order to expose embedded particles of the resin and roughen the resin. However, this method also has the disadvantage that the CRP component is still exposed to high thermal loading during the coating. Moreover, it requires further method steps.